1) Field of the Invention
The field of the invention pertains to communications and, more particularly, to communication protocols for spread spectrum wireless communication systems.
2) Background
Wireless communication systems typically comprise a number of mobile "user stations" or "handsets" and a number of stationary or fixed "base stations" which are capable communicating with each other. Communication between the various base stations and handsets is usually accomplished by allocating communication resources among the handsets and the base stations to establish and maintain communication links between the handsets and base stations. The scarcity of communication link resources, including frequency spectrum, creates a need for the efficient allocation of these resources. While a number of techniques exist for allocating these communication link resources, including frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA), none alone have proven adequate to supply the ever increasing demand for these resources.
In known wireless mobile communication systems, many more user stations are deployed than there are communication resources available for dedicated links at any one time. However, communications (e.g., telephone calls) originating or terminating at any one user station are generally sporadic, and thus it is usually neither necessary nor desirable from an economic or efficiency standpoint to reserve a single communication link for use by each user station as is sometimes done in completely land based systems. Instead, a mobile communication system typically shares communication resources among multiple user stations on an as needed basis. Where communication links are not reserved for each user station, temporary communication links between user stations and base stations are established, and when a particular communication session is completed and the link is no longer needed, the link is dissolved. Examples of such systems include land mobile radio, cellular telephone, and personal communication systems (PCS). In such systems a communication protocol is generally desirable for allocating the communication resources for communication links in response to demand.
In addition to the finite number of communication link resources in a wireless environment, RF interference is a concern and can serve to limit the number of link resources available in a given environment. In the wireless environment, RF interference can come from sources internal to the communication system or external to the communication system, or both. RF interference from internal sources may be caused by, for example, other user stations with communication links in the same or in neighboring cells, by base stations in neighboring cells, etc. RF interference from external sources may be caused by geographically collocated spectrum users operating in the same or neighboring frequency bands, and by remotely located users of the same frequency bands.
To minimize external RF interference in wireless systems, government regulations have tended to constrain the power level of signals generated by user stations and base stations. For example, in a personal communication system (PCS), existing regulations establish limitations on the allowable RF interference from user stations and base stations to fixed microwave users in the PCS bands. There are also government restrictions on the allowable RF interference from user stations and base stations to users of neighboring frequency bands. These constraints create a need for control of transmission power levels and, if possible, transmission directivity while not limiting the communication link resources.
In addition to the foregoing, communication resources are affected by geographical considerations and as such wireless communication systems should be adaptable to operate over a wide variety of changing environments, since direct line-of-sight paths between a user station and a base station are not always possible, particularly in urban areas. Also, multipath propagation conditions may vary substantially between urban and rural settings. Accordingly, a preferred system is robust and capable of accounting for multipath effects.
Another consideration in a wireless communication system is the degree of symmetry of communication traffic between user stations and base stations. In some voice-oriented mobile communication systems, such as presently known cellular telephone systems, communication traffic between the user stations and the base stations is symmetrical during most communication sessions. However, for some communication sessions involving data transmission (e.g., computer data or a facsimile) in some communication systems, communication traffic may be highly asymmetrical. Consequently, it is generally beneficial to have a communication protocol capable of handling asymmetrical communication requirements without placing further demands on communication link resources.
In addition to the foregoing, a wireless communication protocol preferably should operate with small, low cost handsets. It is therefore desirable that communication protocols be compatible with low power communication techniques, so as to extend battery life in handsets.
Spread spectrum communication techniques are well suited for use in wireless communication systems, particularly in a mobile environment. In direct sequence spread spectrum communication, a data signal is encoded with a distinguishing pseudo-random chip sequence, or spreading sequence before transmission. The spreading sequence typically contains many more "chips" than "bits" in the data signal, and thus "spreads" the data signal over a wider bandwidth than that of the data signal alone. At the receiver, the signal is typically "despread" to its original bandwidth by means of a despreading sequence matched to the spreading sequence. An informative discussion of spread spectrum communication may be found in, e.g., R. Dixon, Spread Spectrum Systems with Commercial Applications (J. Wiley & Sons, 3d ed. 1994).
Consequently, it would be advantageous to provide a flexible and robust communication protocol for use in wireless communication systems employing spread spectrum techniques that address the above concerns. It would be further advantageous to provide a protocol suitable for use in personal communication systems operating in a mobile environment. It would be further advantageous to provide a protocol well adapted to use with low-cost, low-power pocket telephones.